The present invention relates to an ion trap, a mass spectrometer, a method of trapping ions and a method of mass spectrometry.
Three dimensional (or Paul) ion traps comprising a central ring electrode and two end-cap electrodes are well known. Similarly, two dimensional or linear ion traps comprising quadrupole rod set ion guide with two end electrodes are also well known.
It is known to mass selectively eject ions from a conventional ion trap in a sequential manner by scanning or stepping the amplitude of an RF voltage which acts to confine ions within the ion trap. Alternatively, the frequency of a supplemental excitation potential which is applied to the electrodes of the ion trap may be scanned or stepped.
Ions having differing mass to charge ratios may be simultaneously ejected from a conventional ion trap by applying two supplementary excitation potentials to the electrodes forming the ion trap. The two supplementary excitation potentials have different frequencies. Ions which are subsequently ejected from the ion trap all follow the same exit route out from the trapping region.
It is known that RF ion traps may be used to contain simultaneously both positive and negative ions. This enables ion-ion interactions to be utilised to effect ion fragmentation or reaction in the gas phase.
The conventional approach of sequentially ejecting ions having differing mass to charge ratios from an ion trap limits the speed at which an analytical scan can be accomplished without degrading performance. It is known that the mass resolution achieved when ejecting ions by resonance ejection or by mass selective instability reduces as the speed of the analytical scan increases.
In general, the mass resolution is proportional to the number of resonance field periods that an ion experiences before it is ejected. Highest resolution is generally achieved when ion ejection occurs over extended time periods with the minimum amplitude of auxiliary excitation potential needed to effect ejection.
The speed at which the frequency or amplitude of a confining RF potential may be scanned or stepped during an analytical scan is also limited by the electronic circuit used. This will also limit the speed of analysis.
Another problem with known ion traps is that it is not possible to mass analyse individually and simultaneously positive and negative ion species from a given ion-ion reaction within the ion trap. Ions having different polarities but having the same or substantially similar mass to charge ratios will have the same frequency of oscillation within the ion trap. Therefore, conventionally, mass spectra may only be produced which correspond with the sum of the positive and negative ions residing in the ion trap without it being possible to distinguish between ions having different polarities.
It is therefore desired to provide an improved ion trap.